PS6-9. The Effect of L-carnitine on Growth, Feed Efficiency, Estradiol Production, and Ovarian Characteristics in Beef Heifers.

Abstract L-carnitine (LC) is an amino acid-derivative that functions primarily in energy metabolism via its role in ß-oxidation. Supplementation of LC has been reported to improve circulating energy metabolites, steroid hormone production, embryo and oocyte quality, and pregnancy rates in multiple species including dairy cattle. In beef cattle, LC research has primarily investigated its effects on growth performance and feed efficiency, reporting equivocal results. No published research has investigated the effects of LC on metrics of reproductive performance in beef cattle. The objective of this study was to evaluate the influence of LC supplementation on serum estradiol (E2), antral follicle counts (AFC), and ovarian morphometrics, as well as growth performance and feed efficiency in developing beef heifers. Eighteen crossbred heifers were individually fed for 45-d in a dry-lot setting. Treatments consisted of a total mixed ration top-dressed with 0.63 kg dried distiller's grains with solubles (DDGS; CON; n = 9) or 0.45 kg DDGS + 100 g/d LC (LC100; n = 9). Treatments were formulated to be isonitrogenous and to meet requirements for 0.54 kg/d of gain. Intake was recorded daily, with body weights (BW) collected on d 0, 14, 28, and 42. Ovarian measurements and AFC were collected on d 35 using transrectal ultrasonography, after which heifers underwent synchronization of estrus following the Select Synch+CIDR protocol. To capture the E2 surge, blood was collected every 4 h beginning 24 h post-CIDR removal and ending 12 h after observation of estrus. Ovaries were examined via transrectal ultrasonography 12 h post-estrus to record dominant follicle (DF) size. Time of estrus served as h 0 for E2 analysis, and heifers not exhibiting estrus were excluded from E2 and DF analysis. Data were analyzed using the MIXED procedure of SAS 9.4. Treatment served as fixed effect and technician as random effect for AFC and ovarian measurements. Repeated measures were utilized for analysis of growth, efficiency, and E2 data, with fixed effects of treatment, time, and treatment × time. There was no significant interaction or effect of treatment on heifer BW or intake (P ≥ 0.22). Average daily gain and feed:gain also did not differ (P ≥ 0.15). Treatment did not influence AFC (P = 0.88) or ovarian height (P = 0.72), and while LC100 heifers tended to have increased ovarian length (P = 0.06), there was no difference in total area of the ovary between treatments (P = 0.24). Effects of treatment and treatment × time did not influence serum E2 production (P ≥ 0.21), and DF size also did not differ between treatments (P = 0.41). Thus, LC supplementation at 100 g/d did not meaningfully affect growth, feed efficiency, ovarian characteristics, or estradiol production when fed to developing beef heifers.